How to interpret a schematic in terms of input/output

Question

I am programmer who studies electronics for hobby (but a serious one, not just for fun). I consider myself to have a reasonable knowledge on digital electronics. For instance, I already described processors, simple GPUs, network cards, RAM controllers etc in VHDL and then to a FPGA. Regarding digital electronics, this is the kind of knowledge that I have so far.

Now, I want to improve my knowledge in analog electronics. So far, I have studied: transistors, bjt amplifiers, opamps, RLC circuits, passive and active filters, simple linear fonts and some classical ICs such as 555 for instance.

But what I still missing is the ability to read and understand an anolog circuit schematic in the following sense: when I see a digital circuit schematic, it is easy to identify where are the inputs and the outputs, how data flow through the circuit and how each stage transforms the input signal. For instance, the following image is easy to reason in terms of inputs and output.

But when reading an analog circuitry schematic, I can't, yet, divide the schematic in blocks/parts by myself even with careful studying. For instance, the following schematic (a SPMS):

Because of so many in series and parallel conections and because current can flow in both ways in some parts of circuits it is hard to me reason in terms of input and output.

So here is my question: is there a way to read and interpret analog circuit schematics in terms of input/output in the same way that is possible to digital schematics (logic gates, for instance)? Or for analog circuitry there is another way of reasoning about the circuit? In other words: is there a systematic way, an algorithmic way, to read and interpret analog schematics or each circuit requires an ad-hoc analysis? Is there an abstraction that electrical engineers use?

What I have tried so far: to reason about a circuit using the abstraction of signals; try to split a circuit in terms of buffers (because of the input/output impedance) but this hasn't worked well so far because: not all circuits have buffers or work with signals. I've also tried to split a circuit by first looking at the IC's and then the discrete components around it. Then I would go on the IC's datasheet and read for instructions there. But this also didn't work for all cases because not all circuits have ICs.

My goals are: given a schematic, try to identify functional blocks: filters, amplifiers etc and; to be able to design (for me this is the most important goal) schematics of such complexity as the SPMS above. So for both cases I need to understand how one stage is connected to another.

I beg your pardon if my question is a little vague. As I said, I am a programmer, not an electrical engineer and I still lack keywords and concepts. If possible, please help me to improve my question.

Answer 1

It's all about pattern identification, and you get that by looking at simpler schematics and then building up from there. Generally, people pick design patterns from a fairly limited set of options and so once you recognise a few you can start to get a sense of what parts of the design do. After that, you can "ignore" those and concentrate your efforts on understanding bits you don't know.

I often find a challenge is the actual layout of the schematic. For example, in your SMPS schematic, it's clearly made to fit tightly into a rectangular space so some patterns are not in their textbook arrangement. Practice, practice, practice, and ask here when you need some help :)

Answer 2

The context to the schematic gives you the initial route into the circuit.

For instance, you have illustrated an SMPS. By definition, this takes a power input, and delivers a regulated DC output. You now have to scan the schematic, until you find the words 'INPUT FILTER' at the top left, and 'DC OUTPUT at the top right.

Within that circuit, there will be many blocks which individually have an input and an output, and are strung together to perform the overall function. Here the part identification is your friend, and after googling them a few times, you will get used to the numbers quickly. The TL494 and the 78L05 are a switch mode controller and a low power voltage regulator respectively. You read the data sheets for those, and they tell you what the parts do, and what the pin functions are.

The examples you've chosen for digital versus analogue are somewhat extreme. If you'd posted the schematic of an MCU, with busses going between RAM and ALU and peripherals and ... you get the general idea, no inputs or outputs obvious unless you know what you're looking for. The analogue equivalent in complexity for your ABC logic function would be somewhere between the TL494 and the 78L05. The SMPS equivalent would be the MCU.

Get used to the numbers, use google for the datasheets, decompose into functional blocks (like an SPI peripheral), start simple, and lots and lots of practice. It's perfectly normal for your head to explode when you get into a new field.

You might have studied opamps and 555s, but have you built anything from them? Electronics really is a practical discipline. If you don't want to get a breadboard and a DMM, then at least play with a circuit simulator. I read programming manuals and think I understand, but it's not until I've got something well beyond 'Hello World' working that I realise I didn't understand, and do now.

Answer 3

I guess you're looking for some kind of flowchart, something that can be evaluated by some algorithm, from start to finish, like a computer program. You expect the schematic to describe what the machine does. But the electrical engineer looks at a schematic to see how the machine is built. He uses experience, context, and other resources to see what it does and how it works.

The digital "circuit schematic" you provide is one step beyond a circuit schematic. Where's the power supply? What kind of IC do I use? Which gates are in which ICs? What are the pin numbers? What about decoupling capacitors? Connectors? ESD protection?

Nothing says schematics at a similar level of abstraction can't exist for analog circuits. For example, here's a very basic software defined radio:

^{simulate this circuit – Schematic created using CircuitLab}

Here's another, a Softrock Lite II:

The only difference is the amount of detail present in the schematic. One distills the circuit into functional blocks with clear inputs and outputs. The other provides a complete description of each physical component in the circuit and how they are connected.

You asked how to interpret schematics in terms of input and output. If it's the kind of schematic that shows functional blocks, it's easy.

But if it's the kind that shows an electronic circuit, it's not generally possible. An electronic circuit is a machine, like a watch or a jet engine or a pencil sharpener. The schematic is like a blueprint: it tells you how to build the machine, not what it does. The schematic is not a flowchart. The electronic components don't run one at a time like instructions in a computer program. They run all at once. A line on a schematic isn't necessarily connected to an input on one end and an output on another end. It doesn't indicate a flow of information. Rather, it merely indicates two parts are "touching", electrically speaking.